In mineral processing, flotation is a process used to recover fine particulate solids contained in mineral pulps or slurries. The particles which are to be removed are treated with suitable reagents to render them water repellent and air or a suitable gas is introduced into the pulp or slurry to form small bubbles. The water repellent particles which come into contact with the bubbles adhere to the bubbles and rise to the surface of the pulp or slurry to form a foam or froth. The foam or froth with adherent particles is removed and the particles are recovered therefrom.
This type of process can be used in applications other than mineral processing, for example in the aeration of water or other liquid in waste water treatment and pollution control, in the separation of oil from water and in the separation of fibrous material from effluent streams.
In this specification, the term "liquid" is taken to include a mineral pulp or slurry.
A flotation process of this nature is to a large extent dependent on a sufficient rate of production of gas bubbles, and on the size distribution of the gas bubbles, which determines the key parameter of the rate of production of bubble surface area.
Amongst a variety of devices suitable for this purpose, it is known to provide a downpipe comprising a substantially vertically extending conduit having an open lower end, liquid supply means arranged to supply liquid under pressure to at least one downwardly facing nozzle located within the upper part of the conduit so as to form a downwardly issuing jet of liquid from the nozzle within the conduit. Air is entrained from the atmosphere and is partly sheared into relatively fine bubbles, and partly simply mixed into the dispersion to form bigger bubbles, and bubble slugs. The conduit may be submerged in a separating vessel containing liquid, wherein the dispersion of the fine bubbles issues from the base of the conduit and rises upwardly to form a froth-liquid interface and a froth layer.
The design of such aerators has some disadvantages. Gas which is entrained as big bubbles or as bubble slugs does not contribute to a sufficiently high extent to the aeration process, because big bubbles contribute less surface to the process than smaller bubbles of the same total gas volume. Another disadvantage of entraining air in the form of big bubbles is that big gas bubbles or slugs will rise upwardly in the conduit as soon as they have left the region of high downwards velocity created by the jet. This gas might be re-entrained by the jet but it is more advantageous to create finer bubbles in the first place rather than having the gas entrained as big bubbles, have it rising upwards and then being re-entrained. The immediate production of fine bubbles will increase the overall gas entrainment rate and gas carrying capacity of the system, and will also increase the bubble surface production rate. Hence the performance of such a prior art device may be limited by the air entrainment pattern of the liquid jet which may produce a significant amount of large bubbles unsuitable for the desired operation.
It is known that a certain amount of turbulence is beneficial to the flotation process as it promotes the contacting of bubbles and particles. However, too much turbulence can pose a problem to the process, as existing bubble-particle clusters may be destroyed. A prior art aerator as described above will have a turbulent environment throughout the major part of the downcomer, dependent on the velocity and diameter of the jet.
It is also known that in a co-current downpipe of the abovementioned kind, the velocity pattern created by the jet, especially in the case of having one single, large-diameter jet in large-diameter downpipes, is such that a high-velocity stream from the jet develops in the downpipe. This may lead to unfavourable residence time distributions, creation of "dead" zones in the downpipe and suboptimal use of the reactor volume.